Electronic packaging generally contains many layers of packages and electrical interconnections. A first layer may connect one or more integrated circuit chips on a substrate carder. A second layer may interconnect one or more substrate carriers on a printed circuit board. Typically, the interconnections between the substrate carriers and printed circuit boards are accomplished by providing rigid metal pins which are brazed onto the substrate carder. The substrate carriers with rigid pins are subsequently plugged into a connector or wave soldered to an array of plated through holes on the printed circuit board.
Advancement in the performance of electronic integrated circuits is constantly increasing the demands on electrical interconnection technology. Circuits that operate at higher frequencies demand lower inductance interconnection pins. Increased circuit densities demand more interconnections per integrated circuit. Due to the changing and increasing demands on integrated circuit interconnections, an assortment of interconnection options are presently available.
A pin grid array (PGA) is a leaded substrate which is typically electrically and mechanically connected to a through hole printed circuit board. The mechanical structure of this configuration is very strong and durable. However, because it is a through-hole leaded device, pin grid arrays cover a large amount of circuit board surface area. The large physical size will also limit the high frequency performance of the integrated circuit due to stray inductance and capacitance inherent to the structure.
The advancements in electronic circuit performance have created a demand for printed circuit boards with higher circuit density. Surface mount technology (SMT) printed circuit boards allow for higher circuit density than possible with previously existing circuit board technologies. SMT printed circuit boards are cheaper to produce than conventional through-hole printed circuit boards. Therefore, the trend is to adapt existing technologies such as PGA's to be compatible with SMT printed circuit boards.
Ball grid arrays are an attempt to develop a technology which will allow PGA's to be mounted on SMT printed circuit boards. Ball grid arrays are constructed by placing solder balls directly on the bonding pads on the bottom surface of the PGA substrate. The solder balls are made from a high lead content solder. High lead content solder is softer and has a higher melting temperature than lead/tin eutectic solder. The substrate is then attached directly on a printed circuit board so that the solder balls are in contact with the conductive pads on the SMT printed circuit board. The attached substrate resides proximately close to the printed circuit board. This configuration is advantageous for space and electrical performance reasons. However, when thermally stressed, the printed circuit board expands and contracts at a different rate than the substrate does. The differential in expansion and contraction rates creates shear forces on the solder joint that will tear the solder joints apart. Therefore, ball grid arrays are not an acceptable solution for attaching PGA's to SMT printed circuit boards unless the substrate is very small, or the substrate has been specially selected to have a thermal coefficient of expansion matched to the thermal coefficient of expansion of the printed circuit board. An optimal solution would solve the problems associated with the stresses created on the electrical interconnection joints and still be adaptable to the placement of PGA's on SMT printed circuit boards.
Another method of electrically connecting pin grid arrays to a SMT printed circuit board is through the use of solder column technology. Solder columns are made of high lead solder which makes them soft. The high lead content also makes the melting temperature of solder columns higher then the melting temperature of eutectic tin/lead solder. The solder columns are cast directly on the pads of the PGA substrate. The casting process is very difficult and expensive. Although solder columns provide for durable electrical interconnection joints because the electrical interconnections are soft and able to absorb the stresses that can develop between the substrate of the PGA and a printed circuit board, solder columns easily bend when handled. The alignment of the solder columns to the pads on the printed circuit must be good enough to ensure that an electrical connection is made when the solder column is soldered to the SMT printed circuit board. If the solder columns have been bent or misaligned during handling, the columns must be realigned before attempting to attach the PGA carrier to a printed circuit board. The fact that solder columns interconnection pins bend easily makes manufacturing SMT printed circuit boards with solder column technology PGA's on them difficult. Any interconnection pins on a PGA that are bent must be straightened before the PGA is placed on an SMT printed circuit board. The process of maintaining straight solder column interconnection pins adds to the manufacturing costs.
An optimal solution to interconnecting PGA's to SMT printed circuit boards should be easy to manufacture and provide for strong interconnection joints. Present technologies address some of the problems associated with attaching PGA's to SMT printed circuit boards, but none of the present solutions are both easy to manufacture and provide strong electrical interconnection joints. Therefore, there is presently a need for a novel technology that will satisfy these electronic circuit technology problems.